The need for distinguishing between friend and foe on a battlefield is of the utmost importance. The identification of forces is critical during operations of armed forces, but it also has civilian aspects, for example, when it is needed to identify and locate travelers under conditions of difficult terrain.
The present invention provides a system and method for locating and identifying forces on a battlefield. Although the system and method is applicable in identifying ground mobile forces, airplanes, or navy vehicles, its most advantageous property lies in the locating and identifying of ground forces, vehicular or infantry forces, particularly when operating in rough terrain and difficult visibility. The following explanation will therefore relate particularly to the aspect of locating and identifying on ground forces, infantry and/or vehicular, in a battlefield. However, it should be kept in mind that the invention is not limited to such an application.
The problem of locating and identifying friendly forces during operations of armed forces is complicated and well known for many years. There are many cases in which friendly forces were identified as foes (or vice versa), resulting in serious losses. Over the years, significant efforts have been made in order to solve this problem. Satisfactory solutions have been provided, particularly in the aerial battlefield. The main solutions that have been applied for aerial IFF are:    1. IFF Interrogation Use of predefined narrow band signals in predefined frequencies for transmission and receiving of IFF signals. A coded interrogation signal is sent to the interrogated object on a first frequency (e.g. 1030 MHz), and the interrogated object responds in another coded signal on a second frequency (e.g., 1090 MHz). The 1030 MHz and 1090 MHz are standard frequencies which have been assigned for civilian and military IFF applications.    2. Radar Interrogation: A tracking radar system sends a signal to a target which responds (or upon request) by transmitting a coded signal in the frequency of the radar (or another frequency), allowing the radar to identify whether the object is a friend or foe. As the radar has a narrow beam, in air applications in most cases only one airplane is found to be within the transmitted beam.
In aerial applications, the density of the objects is low, the ranges are long, and there are no terrain disturbances. Therefore, the existing aerial systems are relatively satisfactory, and operate comparatively well. More specifically, a radar beam preferably “illuminates” one flying object at a time, so that its IFF response can be linked to a specific space location.
The situation on the ground battlefield is much more complicated. The terrain is generally not covered by radar systems, as the terrain conditions do not allow it. In contrast to the aerial situation, in which the airplanes are essentially exposed to radar systems, the operation of ground forces is particularly based on concealed movement, finding firing positions, and identifying targets in the area. Identification mistakes, or navigation errors frequently result in firing on friendly forces. In infantry forces the problem is particularly acute, as these forces frequently move through rough terrain, and/or under difficult visual conditions.
The most important requirement from an IFF system is to provide to the interrogator the location of all the friend forces which are within a firing range, with a very high level of certainty, and not to falsely identify any of the friends as foe, whether the reason for the false identification is resulted from accidental cause or intentional cause. Further important characteristics of a ground IFF system are:
Covertness: The covertness is of particular importance in ground forces, for their survival. Therefore, it is essential that the interrogation signals do not reveal the interrogator location. This is different from the situation of an airplane, which is a large target distinct from its surroundings, transmitting in any case many electronic signals.
Jamming Immunity: It is important that an IFF system for ground forces be invulnerable to disturbances from external sources.
Operation in Any weather Conditions: It is essential for such a system to properly operate in fog, rain, smoke, dust, and under daylight and nighttime conditions.
IFF systems for ground forces exist. Prior art systems can be distinguished by the following categories:
a. Frequency Range for the Transmission/Reception:
                Optical: Such systems are generally laser or infrared operated, and require a line of sight, which does not always exist. Also, this operation is limited to good visibility conditions.        Millimetric Waves: Systems operating in millimetric waves (generally in the range of 30 GHz–300 GHz) also require a line of sight. Furthermore, the resolution of such systems is limited, and they are relatively vulnerable to detection by enemy forces. If a narrow beam antenna is applied for obtaining good resolution, the scanning is required to cover the area, which lengthen the identification time, and might therefore be non-applicable for infantry.        RF and Microwave Systems: Such systems generally operate in frequencies in the range of from several MHz up to a few GHz. These systems suffer from a relatively poor tracking, particularly as the infantry soldier cannot carry a large antenna. Furthermore, such systems are vulnerable to detection by enemy forces, and to masking.        
Regarding the manner of operation, there are systems applying active interrogators and passive responders, systems applying active interrogators and active responders, and systems using an active beacon that transmits continuously, with a plurality of passive receivers.
There are some other systems that apply GPS for location. Each soldier (or vehicle) carries a GPS unit which determines his exact location. The location of each soldier is transmitted to a control center that receives the locations from all soldiers, and upon request, or when necessary, updates a specific soldier with the locations of all others. Such systems are also vulnerable to masking, as the frequencies of the GPS are public and known, are of narrow bands, and of relatively low amplitude. Also, the transfer of locations requires significant communications activity, which is undesirable in a battlefield.
U.S. Pat. Nos. 5,748,891 and 6,002,708 disclose systems for locating that apply UWB (Ultra Wide Band) transmission and reception of coded signals. The accurate range measuring is provided thanks to the very large bandwidth. The coded transmission and reception enables identification of the responding apparatus. The system is based on an accurate range measuring between several base stations. If the number of stations is 5 or more, and all the ranges between the station are known, then it is possible to find their relative locations. The interrogating station applies a procedure calling to at least 4 other stations, identifies them, measures the range to each of them, and receives from them the ranges between them. In this manner the station can provide the relative location of the stations, but not the direction to them. In order to determine the direction to the stations, the direction to at least two stations not being on a same line with respect to the interrogating station must be determined.
The drawback of said system is that a large amount of communication between stations is needed until the interrogator can determine the location of the responders with respect to himself. More particularly, not only the ranges from interrogator to the other stations is needed, but also the ranges between the other stations are required. All these ranges have to be transferred to the interrogator. In battlefield applications, these limitations are very significant.
It is therefore an object of the invention to provide an IFF system for ground applications, particularly for ground forces, most particularly for infantry forces in a battlefield, but also for vehicular forces.
It is another object of the invention to provide location of all friends in a battle zone, with a very high degree of certainty and very low probability of false identification.
It is another object of the invention to provide a ground IFF system which is invulnerable to detection, interrupting, and/or masking.
It is still another object of the invention to provide an IFF system, each apparatus of which can be carried by a single soldier.
It is still another object of the invention to provide an IFF system that is capable of operating essentially in all weather and visibility conditions.
It is still another object of the invention to provide an IFF system that does not require a line of sight in order to determine location and identification of other similar apparatuses.
Other objects and advantages of the invention will become apparent as the description proceeds.